The invention pertains to method and apparatus for making variegated glass in continuous sheet form.
The term, variegated glass, as used herein, refers to ornamental glass made by incompletely mixing two or more differently colored glass components, while they are in a molten state. While still molten, the glass is shaped to a desired form, and annealed, before the individual color components diffuse and blend into a single, homogeneous color tone. Typically, in the finished glass, the starting colors appear as distinct, readily identifiable color domains, in the form of straie, individual streaks, swirls and waves. One popular type of variegated glass is made by using a white opal as a base color, and then mixing one or more different color components into the white opal. The colors of the added components transform the white opal into a multi-colored composite, which when made into sheet form produces a semitransparent or opaque glass suitable for use in such articles as lampshades, window panes, or the like. Light when caused to filter through the glass, produces a highly esthetic appearance due to the random pattern and variety of colors therewithin.
Colored glass of this nature has been successfully made for hundreds of years using a batch process. Although individual techniques may vary somewhat, basically batch processing involves the melting of two or more glass components, of different colors, in separate pots or tanks. When sufficiently molten, the individual components are withdrawn from the pots or tanks, using a ladle, and combined on a flat, preheated steel table, where the mixing, or as it is sometimes called, marvering, proceeds to the satisfaction of the glass smith. On the table, the batch of molten glass is stirred to disperse the starting colors throughout the volume of molten glass, but without overstirring, which might cause the individual color components to diffuse and blend into an undesired, homogeneous, bland tone. Following such limited stirring, the batch of molten glass is rolled into a single, non-continuous sheet or panel. This may be done by rolling the molten glass, while it lays on the flat steel table, using a heated cylindrical roller, manipulated like a baker's rolling pin. Or the batch of molten glass may be scopped up and deposited at the feed side of a pair of counter-rotating, forming rollers which squeeze the glass into an irregularly shaped sheet which emerges from the discharge side of the rollers. One example of this batch processing for making variegated glass is disclosed in U.S. Pat. No. 1,832,491 issued to Claude Locheille.
It will be apparent that batch processing of variegated glass places severe limits on the productivity associated with the manufacture of this type of ornamental glass. The nature of batch processes makes for inefficient use of both labor and equipment, thereby producing a product which must be sold for a higher per unit cost. For these reasons, numerous prior efforts have been directed to improving the efficiency of existing variegated glass manufacturing processes, and such efforts have primarily focused on techniques and equipment for producing sheet glass on a continuous flow basis. However, because of several, subtle but critical, technical difficulties in making the transition from the batch process to a continuous flow process, previous attempts to make that transition have failed.
One of the technical obstacles involves the practical problem of reducing the length of time, in a continuous flow process, that the differently colored glass components reside together in a molten state, prior to the forming and annealing stages. In practicing the continuous process, the various color components of glass are combined and partially mixed while in a molten state. The mixed molten glass is then passed through a pair of counter-rotating forming rollers which press the glass into a continuous, elongate sheet. Unlike batch processing, in which quantities of molten glass can be combined, mixed and formed in limited amounts within a controlled cooling period during which the temperature of the molten glass drops fom the high end of its working temperature range to the low end of such range, the continuous flow process requires higher temperatures over a greater time duration in order to provide an adequate and continuous volumetric flow rate of molten glass to the forming rollers. These constraints in turn result in a prolongation of the "residency time" during which the molten glass components, reside together while flowing toward the forming rollers. The longer "residency time" produces excessive diffusion of the individual color components, i.e., interblending of the colors across domain boundaries, and thereby causing the resulting glass product to have a bland, lackluster appearance. The amount of diffusion, and thus blending of the color components, is a function of the "residency time" and previous attempts to implement continuous flow processes have required such a long "residency time" that the sheet glass produced thereby has been esthetically and commercially unacceptable, when compared to glass produced by a batch technique.
Another factor which has contributed to the lack of success of the continuous process has been found in the manner in which the molten glass is fed to the forming rollers. A common practice in making sheet glass by feeding molten glass between a pair of counter-rotating forming rollers, is to adjust the flow rate of the molten glass upstream of the rollers so that an excess reservoir of glass builds up behind and in contact with the feed rollers. Because of friction effects between the molten glass and the surfaces of the counter-rotating rollers, a strong back eddy of localized, circulating flow occurs within this reservoir of molten glass. The circulating back eddy in turn causes excessive mixing of the molten glass, thereby again producing a more homogeneous, bland coloration of the resulting glass sheet. Additionally, the reservoir of glass that builds up behind the rollers, constitutes a relatively large volume of molten glass within which glass lying near the bottom of such reservoir tends to stagnate, or at least flow at a substantially less rapid rate, than the glass adjacent the surface. The stagnating glass at the bottom of the reservoir incurs disproportionately longer "residency time," and causes the lower surface of the resulting glass sheet to have a particularly bland or "muddy" appearance.
Conversely, any attempt to reduce the "residency time" in prior continuous flow processes has resulted in inconsistent results due to insufficient mixing. For example, the "residency time" can be reduced by simply combining the differently colored glass components at a location immediately in advance of the forming rollers. This, however, results in an intolerable trade-off by reducing the adequacy of the stirring operations that are needed to sufficiently disperse the different color domains. In order to achieve acceptable dispersement, the various glass components must be combined at a location far enough upstream of the forming rollers to permit adequate stirring of the components as the relatively large volume of molten glass flows, continuously, toward the rollers. Without sufficient stirring, large sections of the resulting glass sheet appear as a single, homogeneous color. This is especially true in the case of variegated glass made by mixing one or more non-white color components into a base glass of white opal. Insufficient mixing in that case, results in a glass sheet having large areas of white, which must be either cut from the sheet, or sold as an inferior product. Also, when large areas of one color are left, this necessarily means that the remaining areas of the sheet will exhibit excessive concentrations of the additive color or colors, which also yields a less than desirable appearance.
It will be appreciated that the foregoing technical difficulties are unique to the continuous production of variegated, sheet glass, and do not apply to all continuous processes for making variegated glass. For example, as disclosed in U.S. Pat. No. 1,529,947 issued to I. H. Freese, and U.S. Pat. No. 1,828,226 issued to E. O. Hiller, continuous or semi-continuous processes have been successfully employed in the mass production of molded, variegated glass objects, such as game marbles. However, in such case, the volume of molten glass used in each mold is of such a limited amount, that the various color components of glass can be combined at a location at, or in close proximity, to the discharge spout that charges the molds. Techniques such as those disclosed in the last mentioned patents, are not applicable to continuous flow production of variegated glass in sheet form.
Accordingly, it is an object of the invention to provide method and apparatus for producing, in a continuous flow process, variegated glass in sheet form, in which the resulting sheet product exhibits discrete domains of different colors, occurring in random patterns of swirls, individual streaks, striae and waves.
Another object of the invention is to provide such a method and apparatus which is capable of consistently producing sheet glass of the variegated nature characterized above, in a semi or fully mechanized system, without requiring the skill of a journeyman glass smith in the mixing of the color components.